1. Field of the Invention
The invention relates to a device for determining the volume fraction of at least one component of a multi-phase medium on the basis of the running time of an electromagnetic desired signal emitted in the multi-phase medium, having at least one emitting device for emitting the desired signal into the multi-phase medium, having at least one receiving device for receiving the desired signal, and having an evaluation device for determining the running time of the desired signal between the emitting device and the receiving device. Furthermore, the invention relates to a method for operating such a device for determining the volume of at least one component of a multi-phase medium.
2. Description of Related Art
The volume fraction of a component of a multi-phase medium consisting of multiple components can be of interest in technical processes for many different reasons. For example, in the conveying of multi-phase media, the composition, and thus, the density of the conveyed medium is of interest as a rule, since only then can a conclusion be made as to the current flow rate within a process or to which amounts have been conveyed in which composition within a time unit. As an example, pneumatic conveyors are mentioned here, this is a special type of conveying in which gases—for example, air or nitrogen—are used as a conveying medium. On the other hand, the bulk material to be moved is called the conveyed medium. Normally, conveying occurs through tubes or hoses that are sealed against the exterior. A special application is, e.g., the conveying of pulverized coal, which is used for feeding the firing chamber of coal-fired power plants or blast furnaces.
An important process factor in conveying multi-phase media is the mass flow of the conveyed medium, wherein:
            m      ⁢                          ⁢              Y        ′              =                            ∂          m                          ∂          t                    =              ρ        ·                  V                      Y            ′                                ⁢        
The volume flow VÝ of the conveyed medium is, in turn, described by:
            V              Y        ′              ⁢                  =                            ∂          V                          ∂          t                    =              v        ·        A        ·        ζ              ,wherein v represents the velocity of the conveyed medium, A the cross section surface of the conveying tube and ζ the volume fraction of the conveyed medium.
Regardless of the shown example of a pneumatically conveyed bulk material, it is known from the prior to arrive at the volume fraction of a multi-phase medium—for example a dual-phase flow mixture—by measuring the velocity of propagation of electromagnetic waves in the medium, since this velocity of propagation is known to be material-dependent, namely is dependent on the relative permittivity ∈r and the relative permeability μr of a medium:
  c  =            1                                    ɛ            0                    ·                      ɛ            r                    ·                      μ            0                    ·                      μ            r                                =                  c        0                                          ɛ            r                    ·                      μ            r                              
When the magnetic characteristics of the individual media of the multi-phase medium are negligible, the velocity of propagation of electromagnetic waves result approximately in c0/√∈r..
It is known that the effective relative permittivity of a multi-phase medium, in particular a gas-solid mixture, can be determined using suitable mixture formulas, wherein a component of each mixture formula is the relative permittivity of the individual mixed media as well as its volume fraction of the overall volume. When the velocity of propagation of electromagnetic waves is measured within the multi-phase medium, it is possible to arrive at the volume fraction of the conveyed medium using a mentioned mixture formula, however, this will not be discussed in detail here. Details can be found in technical literature (e.g., Sihvola, A: “Mixing Rules with complex Dielectric Coefficients”, Subsurface Sensing Technologies and Applications, Vol. 1, No. 4, 2000).
It is further known from the prior art that blank value measurement of the velocity of propagation of electromagnetic waves is initially performed for determining the volume fraction of the components of a dual-phase medium, in which the inner space of the conveying tube is filled with a medium having a known permittivity. A conclusion about the effective relative permittivity of the multi-phase medium can then be made based on the change of the velocity of propagation in comparison to the blank value measurement, for example that the medium can be present as a two-phase flow. With this information, the volume fraction of interest of the conveyed material can be calculated. It is known to use a transmission measurement for this sort of calculation (Baer, C., Musch, T., Gerding, M.: “Conceptual Design of a Procedure for Density Monitoring of Pulverized Fuels in Pneumatic Conveying Systems with Microwaves (8-12 GHz)”, Proceeding of the 6th German Microwave Conference, Mar. 14-16, 2011, Darmstadt, Germany).
Here, the volume containing the multi-phase medium is charged with a desired signal emitted by the emitting device, wherein the desired signal passes through the multi-phase medium and is finally received again by a receiving device. If the length of the path is known between the emitting device and the receiving device, the measurement of the running time of the desired signal can lead to the velocity of propagation of the present electromagnetic waves in the form of the desired signal and, from this, finally also to the volume fraction of the components of interest of the multi-phase medium. Interesting information about the transmission behavior of the space filled with the multi-phase medium can be obtained using a network analyzer in a laboratory construction; however, this solution is complex in terms of equipment and thus also expensive. Furthermore, there is the problem that desired signal emitted from the sending device into the multi-phase medium does not arrive at the receiving device in a direct manner through the medium, but rather interference signals are received, which result, for example, from reflections on the walls of a container holding the multi-phase medium or a tube, so that the recognition of the actual desired signal of interest is difficult with the received overall signal.